The technique of voltage-clamping has been used in the study of slow, regenerative responses in the membranes of both chick muscle cells and identified neurosecretory cells in a snail. Both of these cell- types have N-shaped I-V curves which apparently are the basis of their slow regenerative responses. In a number of ways these "slow spikes" resemble the spikes found in vertebrate and invertebrate excitable tissue; however, the time-constants of the ionic conductances in the chick muscle and snail neuron are orders of magnitude longer than those in other tissue. In the chick muscle the slow, regenerative response appears to be the result of an increase in chloride conductance while that in the neurosecretory neuron results from an increase in sodium followed by potassium conductance. Further experiments are in progress to determine which other ions can carry and/or regulate the current through these ionic conductances.